<template>
  <div class="spec-preview">
    <img :src="defaultImg.imgUrl" />
    <div class="event" @mousemove="move"></div>
    <div class="big">
      <img :src="defaultImg.imgUrl" ref="big" />
    </div>
    <div class="mask" ref="mask"></div>
  </div>
</template>

<script>
export default {
  name: "Zoom",
  props: {
    imgList: Array
  },
  data () {
    return {
      // 显示图片的图片默认下标
      defaultIndex: 0
    }
  },
  mounted () {
    this.$bus.$on('syncDefaultIndex', this.syncDefaultIndex)
  },
  computed: {
    // imgList 有可能是一个空数组,那么通过下标获取空数组中的元素再 .imgUrl 时就会报错,所以可以提前使用计算属性处理一下
    defaultImg () {
      return this.imgList[this.defaultIndex] || {}
    }
  },
  methods: {
    syncDefaultIndex (index) {
      this.defaultIndex = index
    },
    move (event) {
      // 添加了移动事件,代表鼠标动
      let mouseX = event.offsetX
      let mouseY = event.offsetY

      // 计算遮罩的位置让遮罩动
      let mask = this.$refs.mask
      let maskX = mouseX - mask.offsetWidth / 2
      let maskY = mouseY - mask.offsetHeight / 2

      // 限定临界值
      if (maskX < 0) {
        maskX = 0
      } else if (maskX > mask.offsetWidth) { // 大盒子的宽度正好为遮罩的两倍,所以当 maskX > mask.offsetWidth 就可以认为已经超出边界了
        maskX = mask.offsetWidth
      }

      if (maskY < 0) {
        maskY = 0
      } else if (maskY > mask.offsetHeight) {
        maskY = mask.offsetHeight
      }

      mask.style.left = maskX + 'px'
      mask.style.top = maskY + 'px'

      // 大图
      let big = this.$refs.big
      big.style.left = -maskX * 2 + 'px'
      big.style.top = -maskY * 2 + 'px'
    }
  }
}
</script>

<style lang="less">
.spec-preview {
  position: relative;
  width: 400px;
  height: 400px;
  border: 1px solid #ccc;

  img {
    width: 100%;
    height: 100%;
  }

  .event {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    z-index: 998;
  }

  .mask {
    width: 50%;
    height: 50%;
    background-color: rgba(0, 255, 0, 0.3);
    position: absolute;
    left: 0;
    top: 0;
    display: none;
  }

  .big {
    width: 100%;
    height: 100%;
    position: absolute;
    top: -1px;
    left: 100%;
    border: 1px solid #aaa;
    overflow: hidden;
    z-index: 998;
    display: none;
    background: white;

    img {
      width: 200%;
      max-width: 200%;
      height: 200%;
      position: absolute;
      left: 0;
      top: 0;
    }
  }

  .event:hover ~ .mask,
  .event:hover ~ .big {
    display: block;
  }
}
</style>